Many industrial applications require the use of shock and vibration isolators to protect installations, machinery and equipment from hostile environments. One of the most challenging environments is on naval ships where it is customary for systems, machinery and equipment to be required to withstand a specific shock and vibration environment imposed by underwater explosive attack and the rigours of naval service. To provide protection of critical or vulnerable systems it is customary to use shock mounts or isolators to perform the function of reducing the acceleration imparted to the mounted item while also controlling displacement within acceptable levels. It is also a requirement to assure the captivity of the mounted item to a specified shock input to prevent it breaking loose and posing a hazard to the crew and other ship systems.
GB 1,057,750 discloses a shock mount that is commonly referred to as an “X type” shock mount. This “X type” mount is a naval shock mount comprising a collection of two or three oval shaped stainless steel bands arranged to form a compound leaf spring, with a viscous damping compound located between the adjacent steel bands to provide constrained layer damping. Two of such leaf springs are arranged in an opposing end to end configuration to create a mount that takes the form of a continuous band. The mount works as a compliant member between an item supported on the mount and the ship deck, bulkhead or equipment foundation, and has its damping controlled by the constrained layer damping material provided between the adjacent steel bands of each leaf spring. Mounts of this type has been in service for fifty years and continue to be used by many navies worldwide.
A more recent innovation to the “X type” mount is disclosed in GB 2 330 642 A whereby a steel mesh is used as the damping medium between the adjacent steel bands of the leaf springs.
At higher levels of shock loading the “X type” mount will further dissipate the shock loading by yielding of the leaf spring members. This highly effective mount type comes in a range of sizes to support masses from 10 kg upwards. The largest available mount in the range will support a mass up to 550 kg. One drawback of this type of mount is that due to their shape, they have a characteristic stiffness and natural frequency in the lateral direction that is asymmetric, with the stiffness in lateral direction along the long axis of the mount being typically one third of that in the lateral direction transversely across the mount.
In practice, this means that where equal stiffness and/or damping is required in both lateral and longitudinal directions, a number of separate mounts are required to be used between an object to be supported and the underlying support surface with the lateral and longitudinal axes of adjacent mounts being oriented orthogonally to one another.
A further drawback of the prior art mounts is that when a mount is pre-loaded at the upper extremity of the advertised supported mass range for a given mount size, and is then subjected to very high shock loads or such loads that may be imposed by the high displacement, low frequency loads characterised by the phenomenon of ship hull girder whipping, there is a risk that the mount will “bottom out”. Bottoming in this context is caused by closure of the gap between the mounts respective upper and lower securing bolts, resulting in the bolts contacting with metal-to-metal impact which in turn imposes high accelerations on to the supported item
It is therefore an object of the present invention to provide an improved mount that mitigates the disadvantages of the prior art mounts.